HYDRAULIC AND MICROBIOLOGICAL CHARACTERIZATION OF REACTORS FOR ULTRAVIOLET DISINFECTION OF SECONDARY WASTE-WATER EFFLUENT

The efficiency of 5 different types of u.v. reactors for disinfecting secondary effluent was tested in continuous experiments lasting 4-10 weeks. The units were hydraulically characterized by tracer studies and microbiologically calibrated by measuring the inactivation of a model organism, for which the F-specific RNA bacteriophage MS2 was used. All units could be described as a number of completely mixed tanks in series in which the inactivation follows first-order kinetics in both the number of organisms and the u.v. dose. The u.v. dose could be calculated from the product of the average retention time and the average u.v. intensity in the reactor. The average u.v. intensity could be calculated using the infinite line source model and the extinction coefficient of the water measured in unfiltered samples, which varied between 90 and 140 m-1 (transmittance 1 cm cuvette between 25 and 40%). From experiments with naturally polluted secondary effluent, inactivation rate constants for the vegetative bacteria E. coli and faecal streptococci of 0.0055 and 0.0029 m2/J (log10-base), respectively, were found. For the bacterial spores of sulphite-reducing clostridia this constant was 0.00063 m2/J and for somatic coliphages and F-specific bacteriophages values of 0.0069 and 0.0023 m2/J, respectively, were found. Enteroviruses were usually inactivated below the level of detection (1 PFU/101.), hence no quantitative results can be presented. The inactivation rate constant for reoviruses was 0.0024 m2/J, very close to the constant for F-specific bacteriophages, which makes the latter organisms, which are relatively easy to count, very suitable as an indicator organism for virus inactivation by u.v. radiation. The inactivation rate constant for a pure culture of MS2 was 0.0046, which is twice the value for F-specific phages as naturally occurring in sewage. This emphasizes the need to design u.v. reactors on the basis of data obtained with effluents rather than laboratory-strains of micro-organisms.